FIG. 1 is a structure diagram of an existing communication system for a cascade type power conversion device. As shown in FIG. 1, each phase of the conventional cascade type power conversion device is composed of a plurality of power modules 1a connected in series. Taking a common 6 KV system for example, each phase thereof comprises about 5 to 6 power modules 1a connected in series, and the number of power modules in the three-phase cascade type power conversion device is as many as 15 to 18. And each power module 1a needs to communicate with the master control system 1b of the cascade type power conversion device so that the master control system 1b acquires the output state information of the power modules 1a and controls the power modules 1a. In order to achieve reliable communication and high voltage isolation between the power modules 1a and the master control system 1b, a pair of independent optical fibers are provided between each power module 1a and the master control system 1b. As a result, a large number of optical fibers are used, which causes that the management of the lines is difficult and the production costs increase.